Collection communication involves both messaging and data transmission. The messaging part is illustrate below 
The details on alams are described under Get Status - Communication. Communication for getFileIDs can either be via the Get File IDs client or be the same as for the Get File IDs client (See Get File IDs Communications). Similar for the getChecksums communication, this can either be via the Get Checksums client or be the same as for the Get Checksums client (See Get Checksums Communication).
Each message is only sent to one pillar.